1. Technical Field
The present invention relates to chip attachment technology, and more particularly, to a process for fabricating corrosion-resistant conductive pads on a substrate, and to the associated structure.
2. Related Art
An electronic chip may be affixed to a substrate by use of an interconnect that electrically couples the chip to a conductive pad on the substrate. The conductive pad typically comprises copper. In wire bonding, for example, a wirebond interconnect from the chip is attached to the pad and serves to electrically connect the chip to the pad. As another example, a controlled collapse chip connection (C4) interconnect (e.g., a C4 solder ball on the chip) is attached to a conductive pad on a substrate that includes a chip carrier. Unfortunately, pads such as copper pads are susceptible to being oxidized or otherwise corroded due to contact between the pad and atmospheric constituents such as oxygen and moisture. Such corrosion jeopardizes the electrical contact between the wirebond and the pad, resulting in failure of the chip to properly perform in an operating environment.
A known process for reducing or eliminating the preceding problem includes forming a protective capping layer of aluminum on the pad. If the pad includes copper, an intervening thin-film layer between the aluminum cap and the copper pad will be needed to prevent a diffusion of the copper material of the pad into the aluminum capping layer. The thin-film layer may including such materials as titanium nitride or tantalum nitride. This process involves depositing a layer of aluminum on the substrate (or on the thin-film layer that is on the substrate), followed by lithographic patterning and etching, or alternatively chemical mechanical polishing (CMP) of the aluminum, to form the capping layer of aluminum on the pad. Unfortunately, these processes are expensive.
Another known process for dealing with the problem is passivating the copper pad with a known corrosion inhibitor such as benzotriazole. This involves immersing the substrate in the liquid corrosion inhibitor to form a protective film on the copper pad. Unfortunately, the film lacks durability because the layer is very thin and thermally decomposes when the substrate is heated to moderate temperatures.
Accordingly, there is a need for a corrosion resistant pad that is durably corrosion resistant and relatively inexpensive to fabricate.
The present invention overcomes the difficulties of the prior art by using a process for forming corrosion-resistant conductive pads on a substrate, comprising the steps of:
providing a substrate having a metal layer with an initially exposed surface; and
forming an electrically conductive and corrosion resistant metallic layer on the initially exposed surface. A metallic layer is a layer comprising an alloy, an unalloyed metal, or a combination of an alloy and an unalloyed metal.
The present invention has the advantage of providing a corrosion-resistant conductive pad on a substrate, wherein the pad maintains its integrity at elevated temperatures. The process of the present invention has the added advantage of being relatively inexpensive in contrast with the more costly current process that forms a protective capping layer of aluminum on a copper pad.